Various transmission mount structures for vehicles are known, wherein a transmission is supported at a single point while an engine, which forms together with the transmission a vehicle power unit, is supported at two points. One example of such known transmission mount structures is disclosed in Japanese Patent Laid-Open Publication HEI-10-309945.
The disclosed transmission mount structure, as illustrated here in FIG. 7, includes a mount member 107 attached at one end to a mount bracket 103 connected to an end of a transmission 101 and, at the other end, to a mount bracket 108 connected to a chassis frame 104 of a vehicle. The transmission 101 is connected to an output end of a transverse-mounted engine 100. A mount member 105 is connected to a front end portion of the engine 100 via a mount bracket 102 and also connected to the chassis frame 104 via a mount bracket 109. Similarly, a mount member 106 is connected via a mount bracket 102 to a rear end portion of a power unit composed of the engine 100 and the transmission 101 and also connected to the chassis frame 104 via a mount bracket 109. At least one end of each of the mount member 105, 106, 107 is made of elastic material such as rubber and forms an elastic vibration isolator.
With this arrangement, the mount members 105, 106, 107 need to isolate both vibrations in a vertical direction caused during traveling of the vehicle and vibrations in a roll direction caused by the acceleration, deceleration and idling of the engine 100 and to limit undue displacement of the power unit (i.e., a combined engine-and-transmission assembly) relative to the chassis frame 104. The “roll direction” is in the direction of rolling motions of the power unit caused by engine roll torque.
When an attempt is made to reduce vibrations in the vertical direction by using the mount member 107 provided at an end of the transmission 101, a normal approach would be enlargement of the cross-sectional area in a horizontal direction of the elastic end portion (vibration isolator) of the mount member 107. With this enlargement of the horizontal cross-sectional area, the spring constant of the elastic vibration isolator increases and an enhanced displacement limiting effect can be expected. However, since the spring constant in the roll direction of the elastic vibration isolator also increases with an increase in the horizontal cross-sectional area of the same elastic vibration isolator, it is no longer possible for the mount member 107 to effectively reduce vibrations and displacement in the roll direction. Thus, it is difficult to achieve a good combination of the spring constant in the vertical direction with the spring constant in the roll direction.
It is accordingly an object of the present invention to provide a vehicular transmission mount structure which is capable of achieving both a noticeable reduction in vibration in a vertical direction and a noticeable reduction in vibration in a roll direction of a power unit composed of an engine and a transmission.